Embodiments of the invention relate generally to a system and method of whole-body magnetic resonance imaging (MRI) screening having a rapid acquisition with high signal-to-noise ratio (SNR) and minimal distortion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Whole-body screening for metastatic tumors has long been an essential tool for the early detection and timely treatment of such oncologic lesions. Conventionally, fluorodeoxyglucose positron emission tomography (FDG-PET) scans or PET/Computed Tomography (CT) scans were the modality for conducting such metastatic tumor screening. However, due to undesirable ionizing radiation emitted during FDG-PET and PET/CT scans, the use of magnetic resonance imaging (MRI) for metastatic tumor screening has seen increased interest in recent years.
The most common sequences used for MR screening have been short-tau inversion recovery (or short-T1 inversion recovery) (STIR), diffusion-weighted echo-planar imaging (DW-EPI), or a combination of these sequences. STIR is a fat suppression technique that exploits the increased transverse relaxation time (T2) of metastatic tumors to aid in distinguishing the tumor(s) from the background signal. DW-EPI uses high diffusion gradient pulses and suitable pulse sequences to acquire an image in which areas of rapid proton diffusion are distinguished from areas of slow diffusion, which is effective in the screening of tumor metastasis due to the restricted diffusion found in metastatic tumors. Unfortunately, both STIR and DW-EPI sequences are signal-to-noise-ratio (SNR) limited, and each technique necessitates multiple signal averages in order to obtain acceptable image data. The need to obtain multiple signal averages leads to an increase in total scan time, thereby increasing the likelihood of image artifacts and a reduction in overall efficiency. Additionally, DW-EPI images are prone to distortion when used in large fields-of-view (FOV), particularly in the coronal plane, and STIR images create bright signal for fluid and blood that can confuse identification of lesions.
It would therefore be desirable to have a system and method capable of whole-body MR screening for metastatic tumors that utilizes rapid acquisition with high SNR and negligible image distortion, while also minimizing background signal (e.g., fat, fluid-filled cysts, cerebrospinal fluid, blood vessels, etc.) so as to increase metastatic tumor conspicuity.